Caroline John, who died earlier this month, was one of the most crucial factors in the survival of Doctor Who, our favorite time-travel TV show. She only appeared in four stories in 1970, but she was instrumental in transforming the show into something that could endure.

In 1969, Doctor Who was on the brink of cancellation, perceived as an outdated adventure show for kids — but instead of cancelling it, the BBC gave it a brand new format. The mercurial Doctor was dragged down to Earth, the stories were more grown up and darker, and the Doctor was paired with a trigger-happy military organization, U.N.I.T. And it was in color, with cooler visuals and insane fashions.

If that transition hadn't worked, Doctor Who wouldn't have made it to 1971. That's why every Doctor Who fan owes Caroline John a huge debt — she carried a lot of the burden of making this more "realistic" take on the Time Lord's adventures. She played Liz Shaw, a Cambridge scientist who's a skeptical outsider in the Doctor's world, and over the course of four adventures she challenges the Doctor a lot more than most of his previous companions put together. Liz wasn't the first companion who could hold her own intellectually — there were Zoe, and arguably Susan, before her — but Liz was the first who was clearly a grown-up and not particularly in the Doctor's shadow.

Among other things, she helped to sell the moral dilemma of how to deal with another intelligent species with an equal claim to our planet, in "The Silurians." She's the brilliant scientist who's closest to the compromised geniuses who go way off the rails in "The Ambassadors of Death." And she does an amazing job playing the dark, fascistic version of Liz Shaw in "Inferno." As much as I love Katy Manning and her character of Jo Grant, it's hard to imagine Jo playing the same role in any of those stories. If you doubt for a second that Doctor Who has grown up a bit and gained more of an edge in season seven, you only have to look at Caroline John's steely performance to know the truth.

As current Doctor Who showrunner Steven Moffat told BBC News, John was "a brilliant actress," who played "not just a sidekick but a scientist in her own right and a match for the universe's number one know-all... The Doctor's companions should never be his assistants - they're the people who keep him on his toes and that's what Caroline did." [BBC News]